Question: Which of these numbers is prime? ${19,\ 39,\ 72,\ 77,\ 81}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 19 are 1 and 19. The factors of 39 are 1, 3, 13, and 39. The factors of 72 are 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, and 72. The factors of 77 are 1, 7, 11, and 77. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 19 is a prime number.